In micro fluidics it has been very difficult to create valves that seal well. In particular, existing designs for valves based on standard MEMS processes tend to be prone to leakage.
It has also been very difficult in micro fluidics to create valves with a reasonably simple actuator mechanisms. As a result, the valves are complicated to open and close as well as to manufacture.
Further, it has been difficult in micro fluidics to create agitators and high force pumps.